Field of the Invention
The aspect of the embodiments relates to an image processing apparatus, an image processing method, and a medium, and more particularly suitable to those for reduction of noise in an image acquired by radiographic imaging.
Description of the Related Art
A technique for visualizing the inside of a subject has been used. According to the technique, the subject is irradiated with radiation typified by an X-ray, and imaging of a radiation component that has been transmitted through the subject is performed to visualize the inside of the subject.
A radiographic apparatus for imaging such a radiographic image generally performs various types of image processing to enhance quality of a captured image. One of the various types of image processing is noise reduction processing for reducing quantum noise due to fluctuations in radiation quantum and system noise generated by a detector and a circuit.
Various methods have conventionally been discussed as methods for reducing such noise. For example, Japanese Patent No. 4679710 discusses a technique for changing a characteristic of a smoothing filter in accordance with a noise characteristic.
In a case where a suitable smoothing operation is performed in accordance with a noise characteristic as discussed in Japanese Patent No. 4679710, the noise characteristic of an image need to be known. Thus, generally, the noise characteristic is also estimated from a relationship between a dose of radiation to be used for photography and noise.
In such a radiographic apparatus, various types of image processing, such as sharpening processing, dynamic range compression processing, and tone conversion processing are often also performed in addition to the noise reduction processing. In this case, as a process sequence, the noise reduction processing is sometimes executed after other types of image processing. In such a case, when the noise reduction is performed, the relationship between the radiation dose and the noise is changed due to an influence of the other types of image processing. Consequently, the noise characteristic is not accurately estimated.
Japanese Patent Application Laid-Open No. 2003-101815 discusses a technique for allowing image processing other than noise reduction processing to be performed subsequent to the noise reduction processing. That is, before the noise reduction processing is performed, other types of image processing are not executed. In this case, a process sequence is restricted, and configuration flexibility is lost. Moreover, Japanese Patent Application Laid-Open No. 2008-17458 discusses a technique for correcting a change in a noise characteristic of an image due to image processing that is other than noise reduction processing and performed prior to the noise reduction processing. According to such a technique, the correction is not accurately made if the image processing to be performed prior to the noise reduction processing is nonlinear processing.